A biphase signal may be conceptualized as including a series of cells each having clock and data information. One type of biphase signal is the biphase-L signal which has a mid-cell voltage level transition providing both clock and data information. The direction of the level transition corresponds to the data information stored in the cell. For example, a positive going transition represents a mark (high level data signal); and a negative going transition represents a space (low level data signal). Additionally, a level transition may be present at a cell boundary. Such a situation occurs whenever adjacent cells encode the same data information. Another scheme for biphase signal encoding is biphase-M. In this encoding scheme, a level transition occurs at each cell boundary; and either a positive or negative going mid-cell level transition occurs when the cell data is a mark.
Various circuits have been suggested for decoding biphase signals. One such circuit is described in U.S. Pat. No. 3,859,631 issued Jan. 7, 1975 to Holmes et. al. and entitled "Method And Apparatus For Decoding Binary Digital Signals". This reference discloses a voltage controlled oscillator synchronized to and running at a higher frequency than the clock frequency of a biphase signal. The oscillator controls a counter which is used in conjunction with logic circuitry to decode the data information. The voltage controlled oscillator operates at a frequency several times the frequency of the incoming biphase signal. Thus, the circuitry must be capable of high frequency operation.